Hall effect sensors.

[Jahee]

I have a basic understanding of the hall effect, however i'm having trouble understanding how it applies in automotive systems, namely sensing wheel rotation in ABS sensors and in the replacement of the distributor in ignition systems.

Can anyone clear this up for me?

 

[PottedMeat]

It's just another sensor. You've got a number of ferrous bolts that pass a hall effect sensor with an integrated magnet. When a ferrous bolt enters the field of the magnet it causes a deflection of the sensor output.

Like this:

http://www.cherrycorp.com/english/sensors/gs1012.htm

 

[Jahee]

I se... But how does extreme deceleration in a wheel cause a ferrous bolt to enter the magnetic field?

And how does this work in a way that replaces the physical connection made between the terminals in a distributor?

 

[PottedMeat]

Originally posted by: Jahee
I se... But how does extreme deceleration in a wheel cause a ferrous bolt to enter the magnetic field?

And how does this work in a way that replaces the physical connection made between the terminals in a distributor?

The magnet has a static field which passes through the sensor and to a volume just beyond the sensor ( maybe a cm or two ). At that point the sensor outputs some value, maybe 5V for this example. When the ferrous bolt passes the area in front of the sensor, the field is distorted though the sensor and it outputs say 1V. So whenever the bolt passes the sensor, the car computer sees a pulse. The number of pulses per some unit time is proportional to the wheel speed. Extreme deceleration would show up as a high pulse rate decreasing to a very slow pulse rate very quickly.

As to the distributor, I've never seen one, but I would guess instead of terminals theres a magnet on the rotor and a hall sensor. When the magnet passes the sensor the engine computer detects it and fires the plug. So no contacts, no physical wear.

 

[Modelworks]

A lot of cars no longer use a distributor at all.
Instead all firing is done by the computer, which uses a hall effect sensor to read back piston position by magnets placed on the cam shaft.


here is a pic of one of the magnets.
http://www.autozone.com/az/cds...64/repairInfoPages.htm
http://www.autozone.com/az/cds...f1/repairInfoPages.htm